The decision to hand £1bn to the Democratic Unionist Party (DUP) in order to prop up Theresa May's government will have to undergo parliamentary approval.

Responding to a challenge by campaigner Gina Miller, the Treasury solicitor, who heads the Government Legal Department, said “expenditure contemplated by the agreement... will have appropriate parliamentary authorisation".

Replying to a legal letter from Miller, who forced the government to hold a parliamentary vote before it could officially trigger its Brexit plans, and the Independent Workers Union of Great Britain (IWGB), Jonathan Jones added: “No timetable has been set for the making of such payments.”

Miller said: “It beggars belief that, neither at the time the government sealed its dubious deal with the DUP in exchange for their votes in the Commons, nor at any point since, has the government made it clear that the £1bn of taxpayers’ money for Northern Ireland could only be handed over following parliamentary approval.”

She added: "When was parliamentary time going to be found to authorise this payment? And did the DUP know that the cheque the government promised to pay might bounce"

It is similarly unlikely that Tory rebels would vote against the payment to the DUP, as it would effectively undermine the already-vulnerable government, but a future debate on the matter will pile pressure onto May.

Separately, the DUP deal is being challenged in a crowdfunded legal case by Green party activist Ciaran McClean, claiming it breaks the promise of impartiality in the Good Friday agreement and breaches the Bribery Act.